1. Field of the Invention
The present invention relates generally to integrated circuit fabrication, and more particularly to fabrication of a recessed-type field effect transistor with an extra-doped channel region to reduce body effect.
2. Description of the Related Art
Field effect transistors are a common component of many integrated circuits. A field effect transistor includes a source and a drain for defining a channel region, a gate insulator, and a gate electrode. The structure and operation of a field effect transistor is by now well known to one of ordinary skill in the art of electronics.
As device dimensions are constantly scaled down, the channel length of the field effect transistor is continually reduced. However, undesired short-channel effects result in the field effect transistor from such reduced channel length.
For preventing such undesired short-channel effects, a recessed-type field effect transistor is formed with the gate electrode filling a trench in a semiconductor substrate. Because the channel length is along the bottom wall and sidewalls of the trench, the channel length is increased for decreasing short channel effects.
For example, U.S. Pat. No. 5,817,558 to Shye Lin Wu (hereafter referred to as “Wu”) discloses such a recessed-type field effect transistor. The recessed-type field effect transistor of Wu includes a gate electrode filling an opening formed within a semiconductor substrate. In addition, an anti-punch through layer is formed below such an opening in Wu for preventing leakage current between the source and the drain.
However, the anti-punch through layer in Wu is formed well below the opening such that the field effect transistor of Wu is still susceptible to undesired body effect. Generally, the body effect refers to variation of threshold voltage of the field effect transistor from bias on the semiconductor substrate. In Wu, the anti-punch through layer does not abut the walls of the opening such that the anti-punch through layer does not prevent the undesired body effect.